The polyimide resins have advantages such as low dielectric constant, excellent film-forming properties on a large substrate, high heat resistance and high chemical resistance and are hence used in wide fields as insulating materials and materials for protective films for electronic parts. In semiconductor devices such as IC, LSI, power diode and GTO thyristor, a polyimide film is used as a layer insulation film, passivation film or junction coating. A polyimide film is used as a layer insulation film or protective film in IC-mounted substrates for computers, thermal heads, line sensors, and high performance substrates for COG type liquid crystal televisions and the like.
In the case of a photoresist, it is separated and removed after its role is accomplished in an etching process. However, a layer insulation film, a passivation film or the like in a semiconductor device remains unremoved as a permanent film, and is hence required to have high electrical and mechanical properties. These films must also resist a high temperature (300-350.degree. C.) applied in a fabrication process of the semiconductor. The polyimide film is suitable for such a purpose. Polyimide varnishes for formation of polyimide films enjoy an increasing demand in a field of electrical insulating coating.
The polyimide film is generally formed by coating a substrate with a varnish of a polyamide acid (polyamic acid), which is a precursor of polyimide, to form a film and then dehydrating the polyamide acid to cyclize it, thereby polyimidating it. In order to selectively provide a polyimide film at a minute part, it is necessary to form a coating film of a polyamide acid on the whole surface of a substrate, form a pattern on the surface with a photo-resist and etch the polyamide acid with hydrazine or the like.
On the other hand, photosensitive polyimide resins the solubility of which is changed by irradiation of light have been developed in recent years. For example, photosensitive polyimide resin compositions obtained by adding a photo-crosslinking agent to a polyamide acid, polyimide precursors obtained by introducing an acryloyl group into a polyamide acid (Japanese Patent Publication Nos. 30207/1980 and 41422/1980), polyimide precursors obtained by introducing an acryloyl group in the form of a salt into a polyamide acid (Japanese Patent Publication No. 52822/1984), and polyimide precursors obtained by introducing an o-nitrobenzyl ester group into a polyamide acid have been proposed. When such a photosensitive polyimide resin (precursor) is used, a pattern is formed by irradiation of light, and the polyimide resin is cyclized by a heat treatment, thereby forming polyimide.
A method of imparting photosensitivity to polyimide itself, not to a polyimide precursor has been recently developed. Since polyimide is difficult to be dissolved in a solvent, it has been proposed, for example, to use an asymmetric diamine, thereby obtaining solvent-soluble polyimide, or to obtain polyimide, which is soluble in a solvent and has photosensitivity, from benzophenone-tetracarboxylic acid anhydride and an aromatic diamine.
However, the polyimide resins (polyimide and polyimide precursors) have been found to have corrosiveness against metals such as copper and copper alloys (brass and the like), which are in use as conductive materials. The corrosiveness is caused by the fact that a polyamide acid as a polyimide precursor contains carboxyl groups, and the carboxyl groups often remain even after its polyimidation. These carboxyl groups react with copper and copper alloys to corrode them.
Accordingly, for example, when a varnish containing a polyamide acid is used to form a layer insulation film for a multilayer distributing board, a corroding action and generation of copper ions occur in copper or a copper alloy coming into contact with the coating film during the process of heat curing (polyimidation) or after the curing, thereby causing various problems such as insulation failure, breaking of wire, short, rust at metal parts, lowing in adhesion of the film and deterioration of physical properties of the film.
When a varnish of a photosensitive polyamide acid is used to form a film of polyimide, the film tend to remain at unexposed portions thereof upon development due to the reaction of the carboxyl group in the polyamide acid with copper or a copper alloy, so that a difficulty may be encountered on the formation of a good pattern in some cases. More specifically, when the carboxyl group in the polyamide acid is reacted with copper, carboxyl copper (copper salt) is formed. The polyamide acid, in which carboxyl copper has been formed, is lowered in its own solubility in solvents, which forms the cause that a film formed remains undissolved at unexposed portions thereof when development is conducted with a solvent after exposure.
The carboxyl copper is oxidized by dissolved oxygen to form a bivalent copper ion. The copper ion forms the causes of not only the reduction in electrical insulating property, but also lowering in the molecular weight of polyimide formed, lowering in the physical properties and adhesion of the film, and the like. It has been reported that when the bivalent copper ion is combined with an acrylic monomer, migration of an electron occurs to conduct radical polymerization (Chem. Commun., 1968, 273). Even in the photosensitive polyamide acids, those having an acryloyl group as a photosensitive group tend to incur the retention of a film formed at unexposed portions thereof upon development under the influence of the copper ion in particular.
In order to improve the adhesion between a substrate and a polyimide film, some proposals have heretofore been made. For example, it has been proposed to provide a layer of a silane coupling agent on the surface of a copper layer, thereby improving its adhesion to a polyimide film (Japanese Patent Application Laid-Open No. 174439/1989), to add a compound selected from among benzotriazole, 1,2,4-triazole, and these derivatives to a polyamide acid, thereby restraining corrosiveness of the polyamide acid (Japanese Patent Application Laid-Open No. 228359/1990) and to add a silane coupling agent to a polyamide acid varnish, thereby improving the adhesion of a polyimide film formed to a substrate (Japanese Patent Application Laid-Open No. 157286/1985).
However, the varnishes containing these compounds involve a problem that their storage stability is deteriorated. In particular, a silane coupling agent is unstable to humidity. Therefore, the polyamide acid varnish, to which the silane coupling agent is added, involves a problem as to storage stability.
In a method of using the photosensitive polyamide acid to form a polyimide pattern on a copper wiring, in some cases, a film of the polyamide acid may have remained undissolved at unexposed portions thereof upon development due to the migration of a copper ion caused by the reaction of a carboxyl group in the polyamide acid with copper, or humidity resistance of a polyimide film formed and its adhesion to the copper wiring may have been deteriorated. Therefore, it has been generally conducted to plate the copper wiring with chromium or the like where the photosensitive polyamide acid is used. In addition, it has been proposed to conduct patterning of a polyamide acid film on a copper layer subjected to a surface treatment by, for example, a method in which the surface of the copper layer is subjected to a surface treatment with a silicon compound having an amino group (Japanese Patent Application Laid-Open No. 242613/1994), or a method in which the surface of the copper layer is treated with oxygen or a chemical to form a film of copper oxide (Japanese Patent Application Laid-Open No. 198559/1993).
Since the method of subjecting the surface of the copper layer to the plating treatment and the method of subjecting the surface of the copper layer to the chemical treatment to form the copper oxide film thereon increase the number of steps, however, their workability and profitability become poor. Therefore, such method are not preferred.
It has also been known to treat the surface of a copper layer with a rust preventive. Many of rust preventives are composed of a basic compound such as an amine compound, a sulfur-containing compound having a coordinating effect on a copper atom, the sodium salt of an organic carboxylic acid, a hydrazine derivative, or a phenol derivative.
However, the amine compound reacts to the carboxyl group in the polyamide acid to reduce its rust preventing effect. As a result, the formation of copper ions can not be completely prevented. In particular, its effect on the photosensitive polyamide acid in which a polymerizable acrylate site is present is insufficient. The sulfur-containing compound, sodium salt of the organic carboxylic acid, hydrazine derivative and phenol derivative are insufficient in rust preventing effect and moreover form the cause of corrosion where a sulfur atom or a sodium atom remains in the resulting insulating film.